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1 Food Chemistry and Nutrition Departments, University College, Cork, Ireland
2 Department of Animal and Food Sciences, University of Vermont, Burlington 05405
The susceptibility of ß-LG and sodium caseinate to proteolysis by pepsin and trypsin was investigated using SDS or urea-PAGE. The effects were studied of heat, urea, and 2-mercaptoethanol on proteolysis.
Native ß-LG was resistant to hydrolysis by pepsin or trypsin because of its compact globular structure. Heat treatment of ß-LG solutions at 90 to 100°C for 5 or 10 min caused changes in the structure or conformation of the protein that rendered it accessible to pepsin and enhanced the extent of proteolysis by trypsin. The susceptibility of ß-LG to proteolysis by pepsin was markedly increased in the presence of urea (3 to 6 M), and the effect was reversible after removal of urea by dialysis. Proteolysis by trypsin was also increased by the presence of 2% 2-mercaptoethanol. Sodium caseinate was very accessible to pepsin without pretreatment and was extensively hydrolyzed at pH 1 to 5 in the presence of 5 M urea (which prevented the protein from precipitation in the isoelectric region); optimal pH was about 2. The activity of pepsin on sodium caseinate at pH 2 was not significantly affected by urea concentration up to about 8 M. The results indicated that the changes in conformation and structure of ß-LG that were induced by heating, reduction, or urea rendered the protein susceptible to peptic hydrolysis.
Key Words: ßbeta;-lactoglobulin • caseinate • proteolysis
Submitted on September 22, 1994
Accepted on April 19, 1995
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